Prior art circuits and methods for generating ramp signals such as useful in magnetic bias signal recording or erase are known to provide linear ramps having a predetermined slope.
In audio magnetic recording it is necessary to provide a "ramp-up" as well as a "ramp-down" signal of a certain duration or length to avoid a disturbing "pop" recorded on the magnetic medium as it is well known in the art. However, in some applications, for example when recording audio signals in video tape recorders, it is also required to obtain an optimum erase or bias signal level within a relatively short time corresponding to one video field or frame interval, that is, within 30 milliseconds or less to facilitate editing. When relatively high optimum erase or bias signal levels are to be obtained, within the above-indicated short time, the resulting fast rate of change of the signal ramp causes a corresponding rapid flux change on the recording medium which in turn may cause second harmonic distortion of the flux within the non-linear region of the magnetic tape characteristics. The steeper is the slope of the ramp within the non-linear region the faster is the flux change and consequently the larger is the magnitude of the recorded disturbing "pop" signal. Therefore, in the latter applications it is desirable to provide a bias or erase signal ramp as long as possible but still within the above-indicated short interval.
An example of a video tape recorder utilizing such prior art audio bias recording and erase circuit is type VPR-2, manufactured by Ampex Corporation, assignor of this patent application. Its bias and erase signal generator circuit is described in VPR-2 Video Production Recorder, Catalog No. 1809384-02, Page 10-7, issued in May 1980. Briefly, that prior art circuit (not shown) has an integrating operational amplifier for providing a desired linearly ramped output voltage in response to an input voltage step. A voltage limiter connected to the integrator limits the output voltage therefrom to a value determined by a master bias level commonly applied as a D.C. reference voltage to all the recording channels. The limited ramped output signal from the integrator is then chopped by a solid state switch at a desired bias or erase signal frequency, and thereafter filtered to obtain a ramped high frequency sine wave signal. The thusly obtained signal is then attenuated by a potentiometer attenuator connected in the high frequency signal path in each respective channel to obtain respective desired optimum levels and thereafter the resulting attenuated signal is applied to a respective bias or erase current amplifier of that particular channel as it is well known in the art.
To illustrate the above-described disadvantages of the prior art circuits, FIG. 1 shows an example of a playback voltage characteristic Vp corresponding to flux remaining on tape after an erase signal has been applied thereto by a prior art audio recording circuit utilized in a video tape recorder. When such a prior art linear ramp signal of a very short duration passes through the non-linear region of the tape it non-symmetrically modulates the high frequency signal recorded thereon and "bumps" such as shown in FIG. 1 are recorded on the tape which, when played back, cause audible "pops" disturbing to the ear. These bumps remain recorded even after a new information signal is recorded on the tape over these bumps, for example during edits. Consequently, following each edit, an audible "pop" remains on the tape.
FIG. 2 shows an example of a prior art erase voltage envelope Ve which may cause the "bumps" such as indicated in the playback characteristic of FIG. 1. The characteristic Ve comprises a linear "ramp-up" portion r, followed by a constant optimum voltage envelope portion V3 and a linear "ramp-down" portion r' as it is known in the art. For example the portion V3 of erase envelope is obtained within 16 milliseconds and typically the "ramp-up" portion r of the signal envelope will pass through the non-linear portion of the magnetic tape characteristics during the first 2 milliseconds, as shown by portion r1 between T0 and T1 in FIG. 2. The non-linear region of the tape characteristic is illustrated in FIG. 2 by a hatched area. The remaining portion r2 of the linearly increasing ramp signal r subsequently passes through the linear portion of the tape characteristic and reaches value V3 at time T3. While the above-described disadvantage of the prior art circuits may be more apparent with respect to erase signal ramps because of the relatively large magnitudes of flux involved, it is also relevant to A.C. bias signal ramps, depending on the optimum signal envelope magnitude and associated time interval in which that magnitude is to be obtained by the linear ramp signal. The duration of the bumps shown in FIG. 1 generally corresponds to the time interval T0 to T1 or T6 to T7 during which the increasing or decreasing tape flux reaches the upper limit of the non-linear region of the tape characteristic as shown at Vn in FIG. 2 or decreases to zero therefrom, respectively. As it is known, the longer the duration of the ramp signal within that non-linear region the larger will be the wavelength of the "bumps" and if a sufficiently long wavelength could be provided it may fall below the audible bandwidth or below the operating range of an associated playback amplifier. However, as mentioned earlier, such a desired long ramp cannot be provided by the prior art circuits when it is also required to obtain an optimum bias or erase signal envelope level within one field or frame interval in video recording applications.